Field of Invention
The present invention relates in general to the technology of flash lamp, and more particularly to a method for taking photos suitable for an extension flash module of a mobile device.
Related Art
With continuous advance in pixels and quality of digital photography, it has become a trend for mobile devices, such as mobile phones and tablets, to carry the function of taking photos. However, these built-in digital cameras have not been able to perform as well as conventional digital cameras under the circumstances of low lighting or backlighting.
Although some built-in digital cameras also carry light-emitting diode (LED) supplement lamp, both the battery capacity of mobile devices and the heat dissipation issue of LEDs seriously confine the volume of fill light provided by LED supplement lamp. When the distance between the object being photographed and the LED supplement lamp exceeds one meter, the LED fill light cannot provide adequate light source to allow photo sensors to be properly exposed.
Xenon high-intensity discharge lamps (HIDs) can provide a large amount of supplementary lighting within a short period of time. Therefore, conventional digital cameras usually carry xenon HID. A charger in a xenon HID converts low-voltage battery power supply into high-voltage power supply and stores it in a high-volume high-voltage capacitor. Operating in coordination with a mechanical shutter, the xenon HID is then triggered at a proper time to convert the electricity stored in the high-voltage capacitor into high-brightness supplementary lighting within a very short period of time so that photo sensors are properly exposed under circumstances of low lighting or backlighting. A xenon HID requires a high-voltage capacitor having a capacitance from dozens to hundreds of μF and able to withstand 300 to 400 volts. In pursuit of lighter, thinner and more compact mobile devices, the very large volume of the capacitor fails to meet the requirements of current mobile devices. Therefore, without increasing the volume and weight of existing mobile devices, extension HID flash modules become a feasible and necessary option.
According to the specifications of the capacitance in high-voltage capacitors and HID lamp tubes, the flash time of HID flash modules lasts from dozens to hundreds of microseconds (μs). How to flash at the right time so that all photo sensors are evenly exposed is another important issue to be solved as to whether extension HID flash modules are a feasible option. Mobile devices which carry a photo-taking apparatus normally adopt complementary metal-oxide semiconductor (CMOS) photo sensors and a rolling shutter instead of a mechanical shutter, as shown in FIG. 1. FIG. 1 depicts a schematic drawing of a rolling shutter in the conventional art. In FIG. 1, every line represents the time during which every line in the photo sensor performs light-sensing operation. Although the length of exposure for every row of photo sensors in a frame is the same, there is a delay between the time when a row of photo sensors begins or ends exposure and the time when its preceding row of photo sensors begins or ends exposure. During the delay time, the photo-taking apparatus reads the exposure data in the photo sensors and completes resetting so as to proceed with the exposure in the next frame. Since photos are taken in different environments and photo-taking apparatuses are also set differently, the exposure time of every row of photo sensors lasts approximately from a few milliseconds to hundreds of milliseconds.
However, due to the limitations of rolling shutters, extension HID flash modules usually fail to locate the optimal flash time and cannot improve the quality of photos.